The present invention relates generally to the field of instantaneous frequency measurement receivers with digital processing, and more particularly to an angle-of-arrival (AOA) solution using a single digital channelized IFM receiver.
The conventional IFM receiver is a radio frequency receiver used primarily in electronic warfare. Its basic function is to measure the frequency of pulsed signals radiated from hostile radar. Generally, it may be said that IFM receivers measure the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple, calibrated delay lines. For instance, the received RF signal is divided and simultaneously introduced into a non-delayed path and a delay line of known length. Since the delayed and non-delayed receiver paths are functions of the input signal frequency, conversion of the phase difference signals to video signals provides signals whose amplitudes are related to phase delay. These signals typically take the form of sin .omega..tau. or cos .omega..tau., where .omega. is the angular frequency or the processed input signal, and .tau. is the known time delay. The sin .omega..tau. and cos .omega..tau. signals are delivered to an encoding network which makes amplitude comparisons of the signals, determines the numerical value of .omega., and generates the digital frequency descriptive word.
An IFM receiver has many attractive features necessary for electronic warfare applications, such as small size, light weight, wide instantaneous bandwidth, and fine frequency resolution.
In a digital receiver, the incident radiation is mixed with a local oscillator signal and down converted to an intermediate frequency (IF). This IF signal is discretely sampled and further processing is done using digital techniques. The frequency of the incident radiation may be determined by performing a discrete Fourier transform on the sampled signal.
The following U.S. Pat. Nos. are of interest.
5,280,288--Sherry et al PA1 5,262,837--Shvy PA1 5,235,287--Sanderson et al PA1 5,198,748--Tsui et al PA1 5,102,219--Skagerlund PA1 4,996,533--May et al
The patent to May et al discloses a method for mapping ocean currents using a linear array of antennas each with its own receiver/digitizer system. The summing and phasing of the signals is done using two successive complex Fourier transforms. The patents to Sanderson et al and Tsui et al disclose the use of sample signals being further transformed and the phase and amplitudes calculated. The patent to Sherry et al discloses the use of Fast Fourier Transform in target detection systems. The patents to Skagerlund and Shvy disclose digitization of the received signals.
In solving the AOA problem, an analog signal is processed by multiple antennas and receivers. The incoming signal has to be phase compared to obtain the angle of arrival as shown in FIG. 1. For such a system to operate properly, the receivers have to be phase matched which means that the phases of all receivers must be properly matched or calibrated. This method can handle only one signal.
It is possible to solve the angle of arrival of multiple signals if the data is received in digital format. FIG. 2 shows the arrangement of the expected data from m different antennas. One would like to receive the digital data from the m different antennas at the same sampling times, thus the data in FIG. 2 is aligned according to the time axis. If one can receive the data as in FIG. 2, the frequency of the incoming signals could be detected by performing a Fast Fourier Transform (FFT) on the data from one antenna with respect to time. Once the primary frequencies are determined, one can find the related amplitudes of the data from the other antennas. An FFT can then be performed on the amplitude values found from the above algorithm. The output of this FFT would give the phase information related to the incoming signals and thus the solution to the AOA problem.
The AOA problem has been solved using multiple receivers, but there a number of calibration issues associated with such a system. For this solution to work, the receivers have to be precisely calibrated. See a related copending patent application by James B. Y. Tsui Ser. No. 08/269,318 filed Jun. 30, 1994, titled "AOA Application of Digital Channelized IFM Receiver". The invention described therein makes use of the principle disclosed in a co-pending patent application by James B. Y. Tsui Ser. No. 08/269,317 filed Jun. 30, 1994, and in a paper by T. W. Fields, D. L. Sharpin and J. B. Tsui titled Digital Channelized IFM Receiver, presented at the IEEE MTT-S International Microwave Symposium, May 24-26, 1994 at San Diego Calif., and published in the Digest of the Symposium. These two copending patent applications and the paper are hereby incorporated by reference.